The present invention relates to a signal transmission system and, more particularly, to a secret or privacy signal transmission system.
Privacy signal transmission systems heretofore proposed may generally be classified into two types, i.e., a spectrum inversion type and a type which allows messages to be exchanged using digitally processed privacy codes (e.g. key codes or PN codes). The spectrum inversion type system is not a perfect privacy implementation, however, since it, inherently allows sound volumes to be identified and even part of the voice to be overheard from which the content of the communication can be reconstructed. In addition, conversations can leak between common channels when there is common channel interference in a radio system with independent receivers of the same type.
The privacy code type system, on the other hand, is free from the possibility of eavesdropping or leak, since conversations are exchanged between only specific individuals which share common privacy codes (e.g. key codes or PN codes). Nevertheless, this system has disadvantages in transmission efficiency and in circuit structure. At a transmitting terminal of the system in question, an audio signal such as voice is quantized to provide a parallel digital signal train. This parallel signal train is scrambled with a key or PN code for privacy, converted into a serial digital signal train, and transmitted to a receiving terminal. At the receiving terminal, the transmitted serial signal train is converted into a parallel digital signal train which is descrambled with the key or PN code. The descrambled digital signal is converted into an analog audio signal.
As can be seen from the foregoing, the privacy code type system inevitably needs a parallel-to-serial (P/S) and a serial-to-parallel (S/P) converters. To convert the serial digital signal train into the parallel one, the receiving terminal also requires frame sync signals. Inserting the frame sync signals into the audio digital signal train degrades the transmission efficiency and requires an inserting circuit for the sync signal at the transmitting terminal and an extracting circuit for the sync signal at the receiving terminal. The P/S and S/P converters and the inserting and extracting circuits make the whole circuit structure complex.